1. Field of the Invention
This invention relates to an audio signal reproducing apparatus and, more particularly, to a small sized audio signal reproducing apparatus in which reproduced audio signals may be heard by a headphone.
2. Description of the Prior Art
Recently, a small sized audio signal reproducing apparatus, in which audio signals may be heard by a headphone, has becomes popular to provide for a more extensive range of appreciation of music or the like.
With the small sized audio signal reproducing apparatus for hearing the reproduced audio signals with a headphone, there is a demand for further reduction in size and weight since such reduction possibly leads to more widespread use and mode of application.
However, with the above described small sized audio signal reproducing apparatus, magnetic tapes or optical disks, such as for example, compact disks, are used as the signal recording media, so that the apparatus in its entirety cannot be reduced in size beyond the size of these recording media. On the other hand, since the mechanical parts for driving the recording medium are necessitated, the operational reliability may be lowered due to wear or damage to movable parts. The playback operation may also be affected by mechanical disturbances, such as vibrations. In addition, there is a limit to reduction in size and weight due to the use of an electrical motor or a plunger.
Recently, with the progress in the technique of high efficiency compression encoding of audio-signals, high-fidelity playback sounds may now be produced even at the rate of the order of, for example, 64 kb per second per channel. Among the techniques for such high efficiency compression encoding, there are a sub-band coding (SBC) in which audio signals on the time axis are divided into a plurality of frequency bands prior to encoding; an adaptive transform encoding (ATC) in which signals on the time axis are converted by orthogonal transform into signals on the frequency axis which are then divided into a plurality of frequency bands, and adaptive encoding is performed in each of these frequency bands; and an adaptive bit allocation (APC-AB), which is a combination of the above described SBC and an adaptive predictive encoding (APC) and which consists in dividing the signals on the time axis into a plurality of bands converting the band signals into base-band signals or low frequency signals and performing plural order linear predictive analyses for predictive encoding.
For compression encoding of audio signals on the left an right stereo channels, it has now become possible to transmit high fidelity stereo audio signals at the transmission rate is the order of 64 kb per second per channel through the use of the correlation between the left and right stereo signal or the use of codes of unequal lengths.